Recently, a semiconductor Mach-Zehnder modulator (SMZM) has been commercialized as an optical modulator. The SMZM is more easily realized as a small device than an LN modulator etc., and has a broader wavelength band characteristic than an electro-absorption modulator (EA modulator).
As a related technique, an optical semiconductor device in which a semiconductor laser and a Mach-Zehnder modulator are integrated in the direction of optical waves guided on the same semiconductor substrate has been proposed (for example, Japanese Laid-open Patent Publication No. 2009-198881).
As another related technique, the following optical modulator has been proposed. That is, the optical modulator includes: optical interference means for branching input light into two optical waveguides, combining light beams which propagate the respective optical waveguides, and outputting the combined light; phase modulating voltage supplying means for supplying a voltage for modulating of the phase of propagated light to at least one optical waveguide in the two optical waveguides; direct current voltage supplying means for supplying a direct current voltage to at least one optical waveguide in the two optical waveguides; and direct current control means for controlling the value of the direct current voltage supplied by the direct current voltage supplying means depending on the wavelength of the input light (for example, Japanese Laid-open Patent Publication No. 2005-326548).
As a further related technique, the following optical transmitter has been proposed. That is, the optical transmitter includes: a light source, a drive circuit for generating a drive voltage depending on an input signal; an optical modulator for modulating the emitted light from the light source depending on the drive voltage, and converting the input signal into an optical signal; and an operation point stabilization circuit for detecting the drift of the operation characteristic curve of the optical modulator, and controlling the optical modulator so that the operation point is placed in a specified position with respect to the operation characteristic curve. The optical transmitter further includes an operation point shift circuit for shifting the operation point by half cycle on the operation characteristic curve according to an operation point switch signal (for example, Japanese Laid-open Patent Publication No. 04-140712).
The SMZM includes a pair of optical waveguides. The input light from the light source is branched and directed to the pair of optical waveguides. In addition, the SMZM also includes an electrode for supplying an electric field to each optical waveguide. A drive signal generated from a data signal and a bias voltage are applied to each electrode. Then the SMZM generates a modulated optical signal by modulating the input light with the drive signal. In this case, a high quality modulated optical signal is generated by appropriately adjusting the drive amplitude (that is, the amplitude of the drive signal) and the bias voltage.
However, the static characteristic of the SMZM indicates variance for each device, and depends on the wavelength of input light. Therefore, to determine in advance the optimum combination of a drive amplitude and a bias voltage for each SMZM while considering the wavelength of input light, an enormously long time is taken. In addition, although the optimum combination of a drive amplitude and a bias voltage is determined in advance for the SMZM, the static characteristic of the SMZM may be changed depending on the ambient temperature, aging, etc. If the static characteristic of the SMZM changes, the quality of a modulated optical signal is degraded. For example, there occur the fold-back of an optical waveform, the degradation of an extinction ratio, the fluctuation of a cross point, the reduction of the aperture of an optical waveform, etc.
The static characteristic of the SMZM is different from that of a common LN modulator. Therefore, although a method of adjusting the operating state of an LN modulator is introduced to the SMZM, it is hard to optimize the operating state of the SMZM.